Polyoxadiazoles (abbreviated as “POD” below) exhibit high chemical and thermal stability as well as high glass transition temperatures. They can be processed directly, for example spun into fibers, or coatings can be made from them.
Polyoxadiazoles are synthesized in various solvents. A known example is the synthesis using oleum (fuming sulfuric acid) as a solvent. Oleum constitutes a very toxic and corrosive synthesis environment. After the end of the synthesis reaction, excessive neutralization of the medium is necessary. The synthesis of polyoxadiazoles in oleum is described, for example, in DD 292 919 A5 and DE 24 08 426 C2.
Iwakura et al. (Y. Iwakura, K. Uno, S. Hara, J. Polym. Sci.: Part A, 1965, 3, pp. 45-54) were the first to demonstrate a method for the production of a polyoxadiazole based on the reaction of a hydrazine sulfate with dicarboxylic acid in polyphosphoric acid (PPA). Nonetheless, the correlations between the properties of the polyoxadiazoles created with various synthesis parameters remained unclear for a long time and also have still not yet been studied in-depth and understood.
RU 2263685 discloses a method for the manufacture of poly(1,3,4-oxadiazole) with molecular masses between 60,000 and 450,000 Da (Dalton, corresponds to g/mole). The method includes a polycondensation reaction of dicarboxylic acid with hydrazine derivatives or with dicarboxylic acid dihydrazide at a temperature of 190° C. to 220° C., which was carried out in a solvent in the presence of triphenyl phosphite for a period of 3 to 7 hours.
JP 63118331 AA discloses a high efficiency method for the synthesis of a polyoxadiazole, in which a dicarboxylic acid and a hydrazine sulfate are condensed using a mixture of phosphorus pentoxide and methanesulfonic acid as a condensation agent.
Polyphosphoric acid is produced during the reaction of phosphorus pentoxide with water and has the general structural formula
with the right term representing phosphoric acid (PA), the left a polyphosphoric acid (PPA) and n a whole number.
PPA is a good solvent for many organic compounds. It is one of the most effective reagents for performing acylation, alkylation, cyclization and for acid-catalyzed reactions. PPA has also proven useful in polymer synthesis. Despite the large number of publications in the field of synthesis applications of PPA, many aspects regarding the effects of PPA as an acid catalyst are not known. Most proposed mechanisms which involve this solvent are not supported by experimental data.
Krongauz et al. (Y. S. Krongauz, V. V. Korshak, Z. O. Virpsha, A. P. Tranikowa, V. Sheina, B. V. Lokshin, Vysokomol soyed 1970; A12:135-139) proposed the hypothesis that the addition von phosphorus pentoxide (P2O5) to the reaction medium during a polymerization in PPA leads to a higher molecular weight of the synthesized polyoxadiazoles. In the presence of P2O5, a viscosity of the synthesized polymer of 2.34 was found; in the absence of P2O5 a viscosity of 1.2. Thus it was concluded that the addition of salt increases the molecular weight. However, the comparison experiments were performed under different reaction conditions, among other things different temperatures and reaction times, so that the influence of P2O5 on the molecular weight of the synthesized polyoxadiazoles is not unambiguously documented by this study.
A method for the synthesis of sulfonated polyoxadiazoles via polycondensation in polyphosphoric acid of hydrazine sulfates with one of a plurality of dicarboxylic acids or derivatives thereof is known from DE 10 2007 029 542.3. This polymer is used for the manufacture of membranes for fuel cells.
A systematic study of the influences of various synthesis parameters on the properties of the synthesized polyoxadiazoles was carried out by Gomes et al. These studies were published in
Gomes et al. (2001): D. Gomes, C. P. Borges, J. C. Pinto, Polymer 2001, 42, 851-865;
Gomes (2002): D. Gomes, PhD Thesis, COPPE/UFRJ, Rio de Janeiro, Brazil, 2002;
Gomes et al. (2003): D. Gomes, S. P. Nunes, J. C. Pinto, C. P. Borges, Polymer 2003, 44, 3633-3639;
Gomes et al. (2004): D. Gomes, C. P. Borges, J. C. Pinto, Polymer 2004, 45, 4997-5004; and
Gomes et al. (2008): D. Gomes, J. Roeder, M. L. Ponce, S. P. Nunes, J. Power Sources 175 (2008), 49-59.
In these studies, among other things, molecular weight, the fraction of residual hydrazide groups and the degree of sulfonation during POD synthesis in PPA were investigated.
Also studied was the influence of the concentration of anhydride P2O5 on the synthesis in PPA while keeping all other reaction variables constant (temperature, time, monomer concentration and molar dilution) and varying the proportions of added P2O5 to the solution medium (Gomes (2002)). In this case it was found that the addition of P2O5 led to an increase of the viscosity of the reaction medium, so that the reaction medium could no longer be stirred in the reaction apparatus. Therefore the homogenization and reproducibility of the results became worse. All experiments were run in triplicate to study reproducibility.
In the publication Gomes et al. (2001), it was shown that the addition of water to PPA resulted in polyoxadiazoles with higher intrinsic viscosity than without the addition of water to PPA. For this purpose, 7 grams of water were added to PPA. This is ten times the amount of water which was produced by the polymerization reaction itself.
One of the factors influencing the reproducibility of the synthesis of polyoxadiazoles is poor mixing at a microscopic level (micromixing), caused by the high viscosity of the reaction medium during the polymerization. The high viscosity of the reaction medium can cause significant fluctuations of the local monomer compositions, which leads to a shortening of the polymer chain length.
It was proposed that the homogenization could be improved by addition of an inert solvent to the reaction medium, so that the properties of the synthesized polyoxadiazoles could be achieved with higher reproducibility under similar reaction conditions. However, it was found (Gomes et al. (2001), Gomes et al. (2004)) that the addition of small quantities of NMP (N-methyl-2-Pyrrolidone) and DMSO (dimethyl sulfoxide) to PPA does not lead to an improvement of the mixing conditions.
Addition of large quantities of solvents in PPA ultimately led to a reduction of the viscosity of the solution. In this last case, however, the polymerization rate was reduced considerably, because the reaction is catalyzed by PPA. Indeed, the addition of small quantities of water to the reaction medium may even contribute to reducing the PPA viscosity, but water can also disrupt the condensation reaction and change the properties of the synthesized polyoxadiazoles.
In further studies (Gomes et al. (2004), Gomes et al. (2008)), experimental conditions were optimized further for reproducible synthesis of high molecular weight polyoxadiazole samples weight, characterized up to 470,000 g/mole, in which polyoxadiazoles were produced in PPA which showed a high solubility in organic solvents and low residual portions of hydrazide groups.
The polyoxadiazole polymers manufactured according to the state-of-the-art generally exhibited an elasticity of about 4,000 MPa, tensile strengths up to 100 MPa and elongations at break of about 14%.